Are vectors passed to functions by value or by reference in C++

In C++, things are passed by value unless you specify otherwise using the &-operator (note that this operator is also used as the 'address-of' operator, but in a different context). This is all well documented, but I'll re-iterate anyway:

void foo(vector<int> bar); // by value
void foo(vector<int> &bar); // by reference (non-const, so modifiable inside foo)
void foo(vector<int> const &bar); // by const-reference

You can also choose to pass a pointer to a vector (void foo(vector<int> *bar)), but unless you know what you're doing and you feel that this is really is the way to go, don't do this.

Also, vectors are not the same as arrays! Internally, the vector keeps track of an array of which it handles the memory management for you, but so do many other STL containers. You can't pass a vector to a function expecting a pointer or array or vice versa (you can get access to (pointer to) the underlying array and use this though). Vectors are classes offering a lot of functionality through its member-functions, whereas pointers and arrays are built-in types. Also, vectors are dynamically allocated (which means that the size may be determined and changed at runtime) whereas the C-style arrays are statically allocated (its size is constant and must be known at compile-time), limiting their use.

I suggest you read some more about C++ in general (specifically array decay), and then have a look at the following program which illustrates the difference between arrays and pointers:

void foo1(int *arr) { cout << sizeof(arr) << '\n'; }
void foo2(int arr[]) { cout << sizeof(arr) << '\n'; }
void foo3(int arr[10]) { cout << sizeof(arr) << '\n'; }
void foo4(int (&arr)[10]) { cout << sizeof(arr) << '\n'; }

int main()
{
    int arr[10] = {1, 2, 3, 4, 5, 6, 7, 8, 9, 10};
    foo1(arr);
    foo2(arr);
    foo3(arr);
    foo4(arr);
}

A vector is functionally same as an array. But, to the language vector is a type, and int is also a type. To a function argument, an array of any type (including vector[]) is treated as pointer. A vector<int> is not same as int[] (to the compiler). vector<int> is non-array, non-reference, and non-pointer - it is being passed by value, and hence it will call copy-constructor.

So, you must use vector<int>& (preferably with const, if function isn't modifying it) to pass it as a reference.

void foo(vector<int> test)

vector would be passed by value in this.

You have more ways to pass vectors depending on the context:-

1) Pass by reference:- This will let function foo change your contents of the vector. More efficient than pass by value as copying of vector is avoided.

2) Pass by const-reference:- This is efficient as well as reliable when you don't want function to change the contents of the vector.